Salutations Milestheold,
Thank you for taking the time to visit the Apple Support Communities! It seems that you are unable to accept a Family Sharing invitation. We will be honored to assist you!
To accept an invitation follow these steps on your iPhone:
“ On your iPhone, iPad, or iPod touch
Go to Settings > [your name] > Invitations.
To accept an invitation, follow the onscreen instructions. When you join a family, you might be asked to confirm your account information and to opt-in to any features or services your family set up. “
If you are unable to accept the invitation your family member can resend an invitation by following these steps:
“ See if your family member accepted the invitation
After you send the invitation, you can check its status under the person's name.
On your iPhone, iPad, or iPod touch
Go to Settings > [your name] > Family Sharing.
Then select that person's name to see the status of the invitation. If you need to resend the invitation, select Resend Invitation. “
For more information see the article below:
Set up Family Sharing
Have a joyful day!